Use Python built-in str.lower in preference to asciiUpper2Lower character table translation

[jayaddison]

This change uses Python's builtin str.lower to perform string lowercasing (typically used during case-insensitive element/attribute name comparison), and provides a nice small parsing performance benefit on Python (cpython 3.9.1).

Benchmark command used: python benchmarks/bench_html.py parse --rigorous

Before

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html_parse_etree: Mean +- std dev: 208 ms +- 14 ms

After

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html_parse_etree: Mean +- std dev: 200 ms +- 12 ms

This follows on from some conversation in #520 regarding comparison operations and lowercasing.

[jayaddison]

Cleaning up some old / stale pull requests; please let me know if this changeset is considered worthwhile and I'll reopen if so.